Recently confronting information society, the information services needed by each subscriber are becoming various. As a result, not only voice services such as conventional telephone and personal communication service but also analog broadcasting signal service such as cable television and data service such as internet are required.
According as contemporary society is developed to information society, these requirements for communication will be more and more various and still more kinds of services will be required.
Meanwhile, in order to satisfy these needs for communication, communication service providers provide services by using various kinds of methods.
The access networks such as ADSL (Asymmetric Digital Subscriber Line) and HFC (Hybrid Fiber Coax) were introduced as the method using wired line and other access networks such as LMDS (Local Multipoint Distribution Service) and WLL (Wireless Local Loop) were introduced as the method using hybrid wireline/wireless access network.
Because these methods have each advantage and disadvantage, according to available transmission media of subscriber, transmission bandwidth, kind of services, initial investment cost of network equipment and so forth, those methods will be suitably used according to given condition of service provider, required condition of subscriber and so forth.
Accordingly, the access network using those access methods mixedly rather than certain access network will be dominant.
The access network for various kinds of communication and broadcasting services is basically composed of either only wired network or complex network using wired and wireless network.
Recently, as optical communication technologies are greatly improved, optical fiber is introduced in wired network. The introduction of optical fiber in wired network is inevitable result in order to overcome physical limitation (limited bandwidth and large signal loss) of the conventional communication network dependent on twisted pair and coaxial cable.
Particularly, as cost down of device and equipment related to optical communication makes optical communication system to be economical, many researches and developments for wired access service using optical fiber have been performed.
Also, these movements are rising to hybrid wireline/wireless access network. The subcarrier multiplexing optical transmission is the technology that transmits analog or digital signals which are carried in carrier by carrying to optical signal intactly.
By utilizing the subcarrier multiplexing optical transmission technology, the conventional wireless base station composed of modulator, demodulator and amplifier can be simply composed of optical-to-electrical converter, electrical-to-optical converter and amplifier. Accordingly, economical construction of wireless base station is possible.
Also, modem and system control part needed for the conventional wireless base station are moved to central office/headend. And because various kinds of functions including communication channel allocation of wireless base station and handoff is performed in central office/headend, the subcarrier multiplexing optical transmission technology can cope with varying communication quantity more flexibly.
According as optical fiber is introduced to wired network and hybrid wireline/wireless access network, the access service of wide band is possible and also centrally controlled hybrid wireline/wireless access network is possible.
However, in spite of these advantages, vast initial investment cost of equipment for optical device and optical fiber network which are still high cost leads to difficulty in construction of economical access service network.
In order to economically provide access service to subscribers, as many as possible parts of optical access network should be owned jointly.
Particularly, because the available bandwidth (>10 THz) of optical fiber is wide enough to accept conventional all kinds of access services, it is anticipated that the minutely described access service can be economically provided if several access services jointly own one optical fiber network by accepting those access services in one optical fiber network.
Accordingly, the development of multi-purpose optical fiber access network which accepts all kinds of access services in one access network and overcomes the current problem of duplicate construction of optical fiber network according to each service has been required.